Add a .github/CONTRIBUTING.md file

The existing Contributing-guide has existed on the wiki for ages,
but it does not get automatic linking by GitHub the way a CONTRIBUTING
file in the repository does. So add that, and have it link to
the original.

Author: jcflack

.github/CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Thank you for your interest in contributing to PL/Java!
+
+First, a quick review of resources you might want to refer to:
+
+PL/Java has a an [Issue Tracker], a [Wiki], a [Mailing List], and of course
+an on-line [documentation site].
+
+PL/Java has had a Contribution Guide for a long time, but over in the Wiki
+where GitHub did not find it for automatic linking. The file you are now
+reading was created so GitHub can find it, and the Conbtribution Guide is
+still over in the Wiki where it's always been:
+
+[Contribution Guide]
+
+Please take a look there, and thank you again for your interest in contributing!
+
+[Issue Tracker]: https://github.com/tada/pljava/issues
+[Wiki]: https://github.com/tada/pljava/wiki/
+[Mailing List]: https://www.postgresql.org/list/pljava-dev/
+[documentation site]: https://tada.github.io/pljava/
+[Contribution Guide]: https://github.com/tada/pljava/wiki/Contribution-guide